The invention relates to vacuum body housings for electron devices.
Historically, electron devices in the first several decades of the 20th century required vacuum tight housings to support the propagation of an electron flux therein. These housings were hermetic structures of various materials and took on a variety of forms requiring a corresponding variety of equipment to fabricate. A very significant part of the cost of any such device was associated with the hermetic-sealed housing. During the last several decades of the century, solid state electron devices evolved for which there was no such vacuum requirement. There remain classes of electron devices which require formation and control of an electron flux in the vacuum environment for which the vacuum tight housing remains a major economic and operational consideration. Typical of these devices are x-ray sources, and image detection devices. Requirements for large scale production efficiencies and increased device complexities motivate an evolutionary approach to the design and fabrication of the package for micro-electronic devices. Generally desirable specifications for the housing would recognize the need to miniaturize the entire package; to assure an inherently low cost for materials and fabrication; to reduce the part count per device; to obtain high yield in the manufacturing process; to employ conventionally available capital equipment; to obtain housings which can be characterized by a standard format; and in appropriate devices, to obtain a satisfactory isolation of any applied high potentials in the miniaturized device scale.
Consider the cooperative benefits of the above enumerated desiderata: a conventional standard form factor may be associated with existing classes of sockets and with existing equipment for surface mounting such devices on printed circuit boards. Unusually added complexities in the form of increased numbers of signal leads can be accommodated in such standard form factors, e.g., plastic leaded chip carrier (PLCC) type socketing hardware. In classic vacuum tubes 8, 12 and 16 leads inserted into the vacuum housing represented a significant level of complexity for the purposes of the device and for its fabrication. Contemporary PLCC sockets accommodate many leads. As many as 128 leads is a common requirement for modem integrated circuits. Such a number of signal and control leads is not unusual for an image detector array, by way of example.
Certain genera of fabrication processes practiced for producing packages for semiconductor devices are employed herein for the novel purpose of achieving vacuum tight housings for microelectronic devices. In the present work, reference will be repeatedly made to the example of a class of image detection devices employing electron bombarded active pixel arrays.
xe2x80x9cTape castingxe2x80x9d is a well known form of fabrication of ceramic objects in the area of semiconductor packages. The term refers to a series of steps and resulting structures, wherein a ceramic slurry is created from selected ceramic precursors and additives for the particular purpose which are mixed on a flat work surface to produce a planar layer for an eventual multi-layered structure. A doctor blade or like instrument is then drawn over the slurry at a selected rate to obtain a uniform material thickness for that component layer. An aperture of specified dimensions is then removed from the interior of the constituent layer. The slurry is then allowed to dry in air and the result is known as a xe2x80x9cgreen tapexe2x80x9d. Depending upon the additives, the green tape is flexible and sufficiently robust to tolerate reasonable handling. The tape is cut to size and a stack of green tape constituent layers is assembled to define a package for housing a semiconductor device. In the context of conventional semiconductor packaging, electrical leads may be printed with refractory metal-based inks deposited on surfaces of one or more component layers to provide electrical communication paths from the interior of the package to the exterior thereof. The stacked green tape assembly is then sintered at selected temperatures of the order of 1500xc2x0 C. to produce a monolithic structure from the multi-layered composite into which the semiconductor chip is mounted, wire bonded to pads connected to the printed leads and the housing is then closed. Tape casting is a well known process for assembling ceramic packages for semiconductor devices. Typical references are xe2x80x9cMultilayer Ceramics: Design Guidelinesxe2x80x9d (Kyocera, CAT/2T9203THA/1242E, 1992) and xe2x80x9cDesign Guidexe2x80x9d (Coors Electronic Package Company, 1998).
In US 5,581,151, a vacuum electronic image detector is known in which a cylindrical housing is formed from a layered ceramic structure, cofired to form a unitary ceramic structure. In this known structure, all control and signal leads (other than the photocathode) are lead through vias to pins downwardly projecting from the base of the housing. The plurality of layers forming this cylindrical known structure define an internal cylindrical cavity comprising a stepped arrangement of sequentially greater (lesser) diameter to support, or form component parts of the structure. Additionally, this prior art achieves a vacuum seal incorporating a flange brazed to the package body to adhere to an indium metal seal to a window, an arrangement that adds cost and processing complexity.
It is known in prior art to employ cold, crushed Indium for vacuum sealing. A representative reference is C.C. Lim, Review of Scientific Instruments, vol. 57, pp. 108-114 (1986).
The present invention exploits use of tape casting to produce vacuum tight composite structures particularly useful for vacuum electronic device housings. In particular, the housing is formed from a laminate of tape casting layers, and a cavity of desired volume is achieved by forming apertures in layers which are stacked upon a first end plate layer which latter directly or indirectly supports at least a portion of the electronic device. Electrode leads are formed on selected pre-fired layers to communicate laterally through the walls of the cavity. Electrical isolation is improved between selected regions of the cavity by varying the dimensions of substantially aligned apertures in non-monotonic fashion to produce an inwardly directed limiting aperture, or alternatively, an outwardly directed cavity extension, or channel. Improved electrical isolation is thus obtained by extending the linear distance on insulating surfaces between ground and high potential, without increasing the external dimensions of the housing. The laterally directed electrical leads also allow for a more axially compact device and permit a vacuum electronic device to conform to form factors commonly applied to semiconductor devices. Inwardly directed structures, separated by a layer of greater outward dimensions, produces a channel. In particular, the channel may be disposed close to a compressive seal and there arranged to capture the extruded flow of a vacuum sealant. The present invention achieves vacuum sealing through a cold, crushed soft metal seal directly between a planar metallized ceramic surface and a closure member.
In particular, the present invention more fully utilizes tape cast housings for vacuum microelectronic devices. A great virtue of the tape cast structure is the freedom of formation of the structural geometry. Another is the monolithic nature of the postfired structure which permits deposit of refractory metal conductive films between component layers thereby achieving electrical communication through a vacuum enclosure without need for insertion of separate feedthrough terminals. Both of these features furnish subtle support for greater efficiencies in resulting vacuum electronic devices. For example, tape cast housings of the present invention are constructed to form internal cavities of generally rectangular cross section which match the generally rectangular form of typical components such as semiconductor circuits or circuit elements realized on semiconductor chips. In the present work, the specific example of an image detector employs an array of diodes sensitive to increments of the electron flux. Such arrays are commonly available in rectangular form. Matching the geometry of the component to the cavity permits a generally smaller cavity resulting in less wasted volume. The smaller internal cavity implies the lesser internal surface area, which is favorable for the ultra high vacuum (UHV) environment to be realized therein.
In like manner, forming conducting paths between the green tape layers provides for distributing signal leads over the lateral walls of the housing in contrast to the practice of bringing all leads through the base of the structure. Accordingly, the inventive housing may be constructed to accommodate well known standards for integrated device sockets (JEDEC type PLCC open frame mounts). A further advantage of laterally extending leads is that the resulting device can exhibit a more compact extension along its principal axis. In the exemplary image detector device described herein, typical applications such as night vision goggles can be formed for wear before the eyes with minimal inconvenience compared with comparable items of prior art.
Aside from the external advantages of a tape cast structure for microelectronic devices, there is an internal advantage in forming consecutive layers having aperture dimensions which do not vary monotonically among a series of layers. Simply, the resulting cavity may be formed to have intruding wall portions adjacent to less intruding wall portions. These serrations can be utilized to provide for added electrical isolation for relatively high voltage conductors without increasing the external size of the package. In like manner, a channel can be formed in the wall of the housing. Such channels are particularly useful adjacent to sealing medial where the compressed sealing media is allowed to flow into the channel for capture therein.
The vacuum microelectronic device is mounted within the tape cast housing and a closure member, including a sealing medium is installed and the seal effectuated in a vacuum environment at normal temperatures. Conventional vacuum preparation of the package includes a baking operation at about 300xc2x0 C. to remove outgassing sources and an electron flux scrubbing to remove adsorbed residual gasses. For UHV microdevices a flat planar member is pressed against a flat metallized receiving surface of the ceramic housing using a soft metal (for example, In) interspersed therebetween and mechanical pressure is applied to the closure member to effect a cold weld between the closure member and the receiving surface. An adjacent channel proximate to the receiving surface receives the flow of the sealant. Providing an edge radius (or other window peripheral detail) to this flat planar member, proximate the ceramic surface where the soft metal extrudes, can improve the seal integrity.
In particular, an image detector is realized within this structure to great advantage. A proximity focused electron flux from a photocathode is intercepted by a CCD or like photodiode array. The image detector device is received in standard type socket hardware (such as a JDEC 68 lead PLCC) and consumes a thickness of about 6 millimeters.